Abstract: A method and apparatus for converting a fluid storage tank susceptible of leakage when a closure cap is improperly applied to a filler spout for said tank. The method comprises replacing the closure cap with an adapter constructed and arranged with securing means sufficiently identical to securing means provided on said closure cap to interfit with securing means provided on said closure spout. The adapter has a one-way valve at the inner end of an adapter pipe, and an enlarged collar beyond the outer end of said adapter pipe that interfits with the filler spout in a manner in which said closure cap previously interfit therewith. The enlarged collar has securing means sufficiently similar to securing means on said closure spout to secure said closure cap to said enlarged collar to prevent entry of debris into said adapter when said adapter has been fixed to said filler spout.